Around 9 or 10 may be too many competitors, but it also depends on the product. If the history consistently shows a steady price and it moves fast enough to where you'll still get a reasonable amount of sales, then you can go forward with products with even more competitors. The main concern with a lot of competitors is potential price drops and not getting enough sales. But if the history doesn't show signs of that, then it could still be worth considering.
The most important factors to consider when determining if there are too many sellers is how fast the product moves, how consistent the price has been, and what the MOQ is.
